FIG. 3 is a block diagram of a conventional vehicle AC-generator controller disclosed in, for example, Japanese Patent Publication No. 3-47058. In FIG. 3, an AC generator 1 includes a rotor winding 102 and three three-phase stator windings 101R to 101T connected with each other in a star-like manner. Diode bridges comprising a full-wave rectifier 2 are provided for each phase of the three stator windings 101R to 101T. Voltages induced in the stator windings 101R to 101T by the rotor winding 102 are full-wave rectified by the full-wave rectifier 2 and applied to a charge terminal B of a battery 8 through an output terminal 201.
An output voltage of the AC generator 1 is adjusted by a voltage regulator 3 in accordance with a charge voltage detected by the charge terminal B of the battery 8. A source voltage is supplied to the voltage regulator 3 by the battery 8 through an externally provided engine key 7 and transistor 5.
The voltage regulator 3 includes a control monolithic IC 303 for detecting a charge voltage through the charge terminal B of the battery 8 to output a field-current control signal having a predetermined duty ratio corresponding to the level of the charge voltage as well as for detecting a no-power-generation state of the AC generator 1 in accordance with an induced voltage applied to the stator winding 101S to output a no-power-generation detection signal, a power transistor 306 having a base adapted to be inputted by a field-current control signal, a grounded emitter and a collector connected to the positive side of the battery 8 through a reversely connected diode, and a power transistor 301 having a base adapted to be inputted by a no-power-generation detection signal, a grounded emitter and a collector connected to an end of a charge lamp 6.
Moreover, when the power transistor 306 is turned on, a field current flows through the rotor winding 102, power transistor 306 and ground from the battery 8. Moreover, when the power transistor 301 is turned on, a current flows from the battery 8 to ground through the engine key 7, charge lamp 6 and power transistor 301 to turn on the charge lamp 6. Resistances 302 and 304 for applying a base voltage to the power transistors 301 and 306 are connected between the bases of the power transistors 301 and 306 and the power-supply line of the control monolithic IC 303.
A transistor 301 constituting an external unit 4 is connected at its collector to the base of the transistor 5 for supplying a source voltage to the control monolithic IC 303 through a resistance 402. An emitter of the transistor 301 is grounded and a signal indicative of the driving state of a vehicle (a driving-state signal) is input to the base of the transistor 301 from various sensors and switches (not shown).
In this case, the operation-state signal denotes a signal such as an on-output signal of an unillustrated starter switch or an engine-cooling-water-temperature signal of an unillustrated water temperature sensor. The external unit 4 inputs an off signal to the transistor 301 for a certain period of time in accordance with each signal in order to reduce the load of an engine when the engine is started. As a result, the transistor 5 is turned off, thereby cutting off the supply of power from the battery 8 to the control monolithic IC 303 and stopping the generation of a field current.
In the operation of a conventional controller, when the control monolithic IC 303 detects a voltage drop of the battery 8 after an engine is started, it turns on and off the power transistor 306 at a predetermined duty ratio to intermittently supply a field current to the rotor winding 102 from the battery 8. By supplying the field current, an induced voltage is generated in the stator windings 101R to lost by a magnetic field generated in the rotor winding 102. Moreover, the generated induced voltage is full-wave rectified by the full-wave rectifier 2, applied to the charge terminal B through the output terminal 201, and charged to the battery 8.
Moreover, when the control monolithic IC 303 detects a voltage drop of the battery 8 immediately after the engine key 7 has been turned on (i.e., immediately after the engine has been started), the monolithic IC 303 turns on the power transistor 306 to supply a field current to the rotor winding 102 from the battery 8 to thereby form a magnetic field in the AC generator 1. In this case, however, because the engine is not operated to run, the AC generator 1 does not generate power, resulting in the no-power-generation state.
When the control monolithic IC 303 detects the no-power-generation state from the stator winding 101S, it drives the power transistor 301 to turn on the charge lamp 6, thus notifying the driver that the battery 8 is currently discharging.
However, when the AC generator 1 starts the power-generating operation by forming a magnetic field in the generator 1 immediately after an engine has started, particularly during a cold period when the function of a battery is deteriorated, the engine rotation becomes unstable due to fluctuation of the generator load torque. Therefore, immediately after start of the engine, the transistor 5 is turned off upon an operation-state signal being inputted to the transistor 301 in the form of the external unit 4 for a certain time until the speed of the engine reaches a predetermined value, thereby turning off the transistor 301 to cut off the supply of power to the control monolithic IC 303. Because the supply of power is cut off, the control monolithic IC 303 stops generating an output to the power transistor 306 to thereby cut off the field current, and hence the AC generator 1 is forced into the no-power-generation state.
Thus, because the source voltage of the control monolithic IC 303 is cut off, no field current is supplied to the rotor winding 102 and the AC generator 1 is brought into the no-power-generation state. However, when the above configuration is used, the transistor 5 having a large current capacity for cutting off a power supply is necessary in addition to the external unit 4 and thus, a large-scale circuit configuration is required.
Moreover, if, for any reason, a leak current is supplied to the power-supply terminal of the control monolithic IC 303 due to a positive potential, a problem occurs in that the power supply cannot be cut off by the external unit 4 and thus, the no-power-generation state cannot be realized.
The present invention is intended to solve the above problems and its object is to provide a vehicle AC-generator controller capable of bringing an AC generator into a no-power-generation state by cutting off a field current in order to reduce the load of an engine upon starting thereof.